Microelectronic modules, such as those containing electronic circuit components, fiber optics, or pressure sensing devices, rely on hermetic sealing, i.e., gas-tight seals, to protect these sensitive components from the corrosive effects of the environment. As is often the case, these modules contain a mosaic of materials having various, and often less than compatible, physical properties.
The art has typically relied upon hermetic connectors disposed through a side wall of the housings of such modules to provide input-output electrical access for cables and the like. In the past, such connectors included a connector body made of a low coefficient of thermal expansion, Fe-Ni-Co alloy, such as Kovar.RTM., and one or more connector pins axially disposed through the connector body. These pins are usually hermetically sealed with glass through small pin-receiving holes disposed through the center of the connector body. The glass insulates the pins from the rest of the connector and is relatively compatible with the Kovar.RTM. base metal, so as to provide an air-tight hermetic fit around the connector pins during severe temperature cycles, such as those experienced by aircraft during flight.
The module housing, for a number of years, was also made of an iron-based alloy, such as Kovar.RTM., which enabled thermal expansion compatibility with the connector during these wide temperature cycles as well as weldability, such as by fusion welding or brazing, for providing a hermetic seal between the connector body and the housing.
With the advent of cost cutting measures, including weight reduction efforts aimed at conserving fuel in the military and commercial aircraft industries, module housings have recently been made from light weight metals, such as aluminum. Although aluminum is easier to machine, is less expensive, and is lighter in weight than iron-based alloys, it is not very compatible with Kovar.RTM., both in regard to weldability and thermal expansion.
In an effort to accommodate the use of aluminum housings in microelectronic modules, the art has resorted, in certain instances, to plating the connector-receiving window regions of aluminum housings and the matching surface of the Kovar.RTM. connectors with nickel, or a similar metal, and then soldering or brazing the nickel-plated surfaces together. Since these metal joining techniques require heating the plated metal surfaces to at least about 200.degree. C. (and as high as 360.degree. C.), and since aluminum and Kovar.RTM. have drastically different coefficients of thermal expansion, stress is created in the joint upon cooling the module to room temperature. Such stress can lead to joint failure and a loss of hermeticity when the module component undergoes subsequent manufacturing operations or is placed in service, especially if such service requires exposure to severe temperature cycling, for example, in aerospace applications.